http channel deadconn detection

[MoonShining]

What this PR does:

Which issue(s) this PR fixes:

Fixes #285

1. https://datatracker.ietf.org/doc/html/rfc7540#section-8.1.2.6 clarified content-length header is optional, so just drop it
2. add http connection close check

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov]

Codecov Report

Merging #291 (3c672f4) into main (32c4ed5) will decrease coverage by 0.07%.
The diff coverage is 33.33%.

❗ Current head 3c672f4 differs from pull request most recent head 0c999c6. Consider uploading reports for the commit 0c999c6 to get more accurate results

@@            Coverage Diff             @@
##             main     #291      +/-   ##
==========================================
- Coverage   56.73%   56.65%   -0.08%     
==========================================
  Files          48       48              
  Lines        2057     2058       +1     
==========================================
- Hits         1167     1166       -1     
- Misses        768      769       +1     
- Partials      122      123       +1     

Impacted Files	Coverage Δ
pkg/grpc/api.go	72.39% <33.33%> (-0.30%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 32c4ed5...0c999c6. Read the comment docs.

[wenxuwan]

LGTM

[wenxuwan]

LGTM